1. Field of the Invention
The present invention relates to an optical head device and an optical information device for recording and/or reproducing information on and/or from an information recording medium such as an optical disc or an optical card.
2. Description of the Background Art
In a conventional optical head device, a part of light reflected and diffracted from an optical disc as an information recording medium is diffracted and the diffracted light is detected by a light receiving portion different from the one for receiving a transmission light (0th-order light), which is not diffracted, and used as a tracking signal (see, for example, FIG. 25 of Japanese Unexamined Patent Publication No. 2004-281026). FIG. 36 is a diagram showing the construction of a conventional optical head device 100 disclosed in the prior art document.
In FIG. 36, a light beam emitted from a semiconductor laser 101 is converted into a collimated light by a collimator lens 102, reflected by a beam splitter 103 and incident on an objective lens 104 to become a convergent light. This convergent light is irradiated to an optical disc 105. The light reflected and diffracted by an information layer 106 of the optical disc 105 passes through the objective lens 104 again and further passes through the beam splitter 103. The objective lens 104 is moved in an optical axis direction and a direction normal to a track by an actuator 107. The light beam having passed through the beam splitter 103 is incident on a holographic element 108 to be partly diffracted, thereby becoming a 0th-order light 110 that is not diffracted and a 1st-order light 111 that is diffracted. The light beam having passed through the holographic element 108 passes through a detection lens 109 to be incident on a photodetector 120.
FIG. 37A is a diagram showing the region division of the holographic element 108 shown in FIG. 36. Dotted line 130 in FIG. 37A shows a beam diameter and the overlap of diffracted lights from the track on the holographic element 108 when the focal point of the objective lens 104 is adjusted to a desired information layer of the optical disc. The holographic element 108 is divided into seven regions 140 to 146 by dividing lines 131 to 136. It is assumed that the region 142 is a first main region; the region 144 a second main region; the regions 140 and 145 a first subregion; the regions 141 and 146 a second subregion; and the region 143 a central region.
FIG. 37B is a diagram showing the arrangement of light receiving portions of the photodetector 120 shown in FIG. 36. The 0th-order light not diffracted by the holographic element 108 is received by a four-divided light receiving portion 150 on an optical axis to detect a focus signal and an RF signal. The 1st-order light 111 diffracted by the holographic element 108 is received by light receiving portions 151 to 154 according to the region division of the holographic element 108. A light beam 161 diffracted by the first main region is received by the light receiving portion 151; a light beam 162 diffracted by the second main region by the light receiving portion 152; a light beam 163 diffracted by the first subregion by the light receiving portion 153; and a light beam 164 diffracted by the second subregion by the light receiving portion 154.
However, in the construction of the conventional optical head device 100, stray lights from other layers (other-layer stray lights) are incident on the light receiving portions for the diffracted light if interlayer thicknesses vary in a multilayer optical disc having three or more layers. Thus, there has been a problem that an offset is added to the tracking signal and no stable tracking control can be performed.